Cation-excess magnetite (Fe3O4-delta) has been found to form by passing H-2 gas through magnetite powder at 300 degrees C with its spinel structure retained. Cation-excess magnetite is’ a metastable phase in the transformation of magnetite into alpha-Fe. The lattice constant of the cation-excess magnetite was enlarged to a maximum value of 0.8407 nm, which is substantially larger than that of stoichiometric magnetite (a(0) = 0.8396 nm). The formation mechanism of cation-excess magnetite in the spinel structure was studied using two specimens of magnetite crystals with (111) and (100) planes developed. They were referred to as (111)- and (100)-magnetite. The formation of alpha-Fe was decreased over the former, where wustite and cation-excess magnetite were formed while keeping the fee arrangement of the oxide ions in the solid. On the other hand, alpha-Fe was more easily formed over the latter. It is considered that the cation excess state can appear when lattice oxygen is removed faster than the formation of Fe2+ ions coming from Fe3+ ions through the H-2 reduction and that the cation excess state is stabilized due to electron hopping between Fe2+ and Fe3+ ions in the B site of the spinel structure.